In an effort to reduce the amount of pollution emissions from gas-powered turbine engines, governmental agencies have enacted numerous regulations requiring reductions in the amount of oxides of nitrogen (NOx) and carbon monoxide (CO) produced. Lower combustion emissions can often be attributed to a more efficient combustion process, with specific regard to fuel injector location, airflow rates, and mixing effectiveness.
Early combustion systems utilized diffusion type nozzles, where fuel is mixed with air external to the fuel nozzle by diffusion, proximate the flame zone. Diffusion type nozzles historically produce relatively high emissions due to the fact that the fuel and air burn essentially upon interaction, without mixing, and stoichiometrically at high temperature to maintain adequate combustor stability and low combustion dynamics.
An enhancement in combustion technology is the concept of premixing fuel and air prior to combustion to form a homogeneous mixture that burns at a lower temperature than a diffusion type flame and thereby produces lower NOx emissions. Premixing can occur either internal to the fuel nozzle assembly or external thereto, as long as it is upstream of the combustion zone. An example of a premixing combustor has a plurality of fuel nozzle assemblies, each injecting fuel into a premix chamber where fuel mixes with compressed air from a plenum before entering a combustion chamber. Premixing fuel and air together before combustion allows for the fuel and air to form a more homogeneous mixture, which, when ignited will burn more completely, resulting in lower emissions. However, the thoroughness and completeness of the mixing and resulting burning of the fuel-air mixture depends on the effectiveness of the mixing.